Radio frequency (RF) power amplifiers are used to amplify RF signals for transmission over the air. One common type of RF power amplifier is a quadrature power amplifier, also known as a balanced power amplifier. FIG. 1 illustrates a conventional quadrature power amplifier 10. The conventional quadrature power amplifier 10 includes a quadrature splitter 12, an in-phase amplifier 14, a quadrature amplifier 16, and a quadrature coupler 18. The quadrature splitter 12 includes an RF signal input node 20A, an isolated node 20B, an in-phase output node 20C, and a quadrature output node 20D. The RF signal input node 20A is configured to receive an RF input signal RF_IN for amplification thereof. The isolated node 20B is coupled to ground via an isolation impedance Z_IS. The in-phase output node 20C is coupled to an in-phase amplifier input node 22A of the in-phase amplifier 14. The quadrature output node 20D is coupled to a quadrature amplifier output node 24A of the quadrature amplifier 16. The quadrature coupler 18 includes an in-phase input node 26A, a quadrature input node 26B, an isolated node 26C, and an RF signal output node 26D. An in-phase amplifier output node 22B of the in-phase amplifier 14 is coupled to the in-phase input node 26A. A quadrature amplifier output node 24B of the quadrature amplifier 16 is coupled to the quadrature input node 26B. The isolated node 26C is coupled to ground via an isolation impedance Z_IS. The RF signal output node 26D provides an amplified RF output signal RF_OUT.
In operation, the RF input signal RF_IN is split into an in-phase signal RF_IP and a quadrature signal RF_Q by the quadrature splitter 12. Those skilled in the art will appreciate that the in-phase signal RF_IP is equal to the RF input signal RF_IN multiplied by √{square root over (2)}/2 and the quadrature signal RF_Q is equal to the RF input signal RF_IN multiplied by √{square root over (2)}/2 and shifted in phase by −90°. Due to the finite input impedance of the in-phase amplifier 14 and the quadrature amplifier 16, a portion of the RF input signal RF_IN is reflected back towards the quadrature splitter 12 by them both. Those skilled in the art will appreciate that due to the nature of the quadrature splitter 12, the reflected signals cancel one another at the RF signal input node 20A while summing at the isolated node 20B, where they are diverted via the isolation impedance Z_IS to ground. Accordingly, these reflected signals do not degrade the performance of the conventional quadrature power amplifier 10.
The in-phase signal RF_IP is amplified by the in-phase amplifier 14, while the quadrature signal RF_Q is amplified by the quadrature amplifier 16. The amplified in-phase signal RF_IPA and the amplified quadrature signal RF_QA are provided to the in-phase input node 26A and the quadrature input node 26B, respectively, where they are recombined at the RF signal output node 26D. Similar to the quadrature splitter 12, portions of the amplified in-phase signal RF_IPA and the amplifier quadrature signal RF_QA from the outputs of the in-phase amplifier 14 and the quadrature amplifier 16 are cancelled at the RF signal output node 26D and summed at the isolated node 26C, where they are diverted via the isolation impedance Z_IS to ground. Accordingly, these reflected signals do not degrade the performance of the conventional quadrature power amplifier 10.
As RF technologies continue to evolve, RF power amplifiers are subject to increasingly strict specifications. For example, fifth generation (5G) wireless standards provide both very stringent linearity and efficiency specifications. Standing alone, a conventional balanced amplifier such as the one discussed above cannot satisfy these stringent specifications. While feed-forward and feedback techniques have been implemented in the art in an effort to increase linearity, they generally degrade other aspects of performance such as efficiency or add significant complexity to a system, which may preclude the use of the system in a mobile device. Accordingly, there is a need for an improved RF power amplifier that provides both high linearity and high efficiency.